with all the zombie/infected blood and body fluids flying everywhere when you see protagonists use knife to get close and personal to finish off enemies, how could they not get sick? they are already shown to have cuts on their skin during the cut scenes where viral blood splashed onto them.
well beside jill anyway, sherry counts because she is not immune to c-virus. --- The Legendary Iron Fisting. Personal best single combo video for Ninja Gaiden. http://youtu.be/szF_7c-eVIM
They're clearly superhuman and imortal. I mean, Helena got bitten by that zombie shark thing on Chapter 3 and she didn't even bleed. It's the protagonists that infect the virus, not the other way around.